The GitHub repo rambo/TinyWire contains an improved version of the USIi2c TinyWireS library I2C TinyWire Slave for Arduino, released under the GNU GPL license.

The new version fixes a few bugs and, most important, adds support for the small ATtiny44/84!

I created a small .ZIP package for this library which can be installed directly from the Arduino IDE (Sketch –> Include Library –> Add .ZIP Library…): TinyWireS.zip library (200 kB).

Feel free to use it and happy coding!